You don't say what year your trailer is.
16s were equipped with 4 lug 13" wheels but I am going to assume that your trailer is less than 30 years of age. That being the case, the standard wheel will be a 13" wheel which is 5 on 4.5 with zero offset, with an optional 14" available.
Most likely you will have a13" wheel with a 175/80 tire.
The best answer, if possible of course, is to call the seller and have him read the size from the side wall.